Formation of crystalline Si_1-xGe_x top layers by ion implantation in crystalline silicon

Semiconductor nanocrystals incorporated in a dielectric film are widely studied as potential candidates to exceed the Shockley-Queisser theoretical conversion limit of photovoltaic cells. In this context, Ge nanocrystals embedded in SiO₂ films seem to be among the best candidates. However, the charges generated in the dielectric film are hard to collect. For this reason, it would be better if the charges were generated in a semiconductor matrix such as silicon, which has better conductivity. However, implanted Ge atoms have poor mobility in a silicon matrix and thus Ge nanocrystals formation is not likely. But even if the formation of Ge nanocrystals seems difficult, it would still be interesting to form crystalline Si_1-xGe_x alloys. This work investigates the formation of the Si_1-xGe_x films by ion implantation and their crystallinity. 36 keV Ge ions were implanted in crystalline Si substrates, with fluences ranging from 5 × 10¹⁵ to 1.5 × 10¹⁷ Ge/cm² at temperatures up to 600 °C. Rutherford Backscattering Spectrometry (RBS), Raman Spectroscopy, and Transmission Electron Microscopy (TEM) were used to investigate the microstructure of the Si_1-xGe_x alloys. It is shown that germanium is mostly incorporated in the crystal network in substitutional sites. XRD, Raman spectroscopy, and TEM confirm that the Si_1-xGe_x layer on top of the c-Si substrate is monocrystalline. TEM also indicates the possible presence of nanostructures, extended defects or both. Implantation was also carried out at temperatures up to 600 °C, with the objective of preserving the crystallinity and promoting Ge diffusion into nanoclusters. RBS shows that the Ge profile is more extended in depth for the sample implanted at 600 °C, compared to a room temperature implantation. As the energy of the ions is the same in both samples, this indicates that Ge is able to diffuse in depth during the implantation at 600 °C compared to implantation at ambient temperature. However, RBS/C shows that the minimal yield is higher for the implantation at 600 °C, indicating a high concentration of interstitials or that crystallinity is deteriorated, as confirmed by TEM.